Research has suggested that greater social engagement [unreadable] a term used to refer broadly to both quantitative and qualitative aspects of social interaction [unreadable] is associated with significantly lower risks for cognitive decline and dementia in older adults. The question of whether and how social engagement may begin to affect adult cognition earlier in the life course remains unanswered, as do questions regarding which aspects of social engagement may be most influential, and whether these effects reflect cumulative histories of social engagement or only more current levels of engagement. Drawing on rich longitudinal data about social engagement (from childhood relationships with parents through current adult relationships) from the Mid-Life in the US (MIDUS) I and II studies, as well as innovative new MIDUS biological and cognitive data, this application proposes a series of analyses designed to significantly advance our understanding of how histories of social engagement may impact on adult cognitive functioning, expanding on prior research to answer the following questions: 1) Considering both quantitative/structural and qualitative aspects of social engagement, which features are most consequential to adult cognition?;2) Are social engagement influences cumulative (i.e., does historical information extending potentially as far back as childhood relationships with parents help to account for observed differences in adult cognitive functioning)?;3) Are histories of social engagement related to adult cognition in younger and middle-aged (as well as older) adults and, if so, are the relationships similar or different at younger versus older ages?;4) Are there sex or socio-economic (SES) differences in the patterning of the relationship between social engagement and cognition?;5) To what extent is the relationship explained by frequently hypothesized mediators such as health behaviors and/or health conditions?;and 6) to what extent do major biological regulatory systems appear to mediate the relationship between social engagement and adult cognition (i.e., how does social engagement [unreadable]get under the skin[unreadable])? Answers to these questions will serve to significantly extend our understanding of whether and how social engagement may influence cognition from younger through older adulthood, and how these relationships may be moderated by age, sex or SES. Only through greater understanding of the intricacies of these relationships can we hope to be successful in any future efforts to reduce risks of cognitive decline and/or impairment for those made more vulnerable as a result of less advantaged social histories.